Growing degree days calculator

What are growing degree days?

Growing degree days (GDD, also called heat units) measure how much warmth a day offers a plant or insect that develops only when it is warm enough. Each day's value is added to the total since a start date such as planting, and growth stages tend to arrive at particular totals even when the calendar dates differ from year to year. Corn that goes in warm soil in April and corn planted in late May reach silking at similar GDD totals but weeks apart.

What is the GDD formula?

GDD = (daily high + daily low) ÷ 2 − base temperature, never below zero. The University of Kentucky's corn guide uses a base of 50°F, limits the high to 86°F and the low to no less than 50°F. Its first example: high 75, low 55, so (75 + 55) ÷ 2 − 50 = 15 GDD. Its second: a high of 97 counts as 86, with a low of 66 that gives (86 + 66) ÷ 2 − 50 = 26 GDD. In Celsius the same method uses a base of 10°C and a limit of 30°C, since 50°F is 10°C and 86°F is 30°C.

What is the difference between the simple average and the modified method?

The simple average subtracts the base from the mean of the high and low and stops at zero. The modified method first resets any high above the upper limit to the limit and any low below the base to the base. The difference shows on cold nights and hot days: a day with a high of 60 and a low of 40 gives 0 by simple average but 5 with the low raised to 50, because the 60-degree afternoon was warm enough to grow; a day of 97 and 66 gives 31.5 by simple average and 26 with the cap, since the plant does not grow faster above 86. Use the method your crop table was built with.

How many GDD does corn need?

It depends on the hybrid. The University of Kentucky says most hybrids grown in the state need about 2,700 to 3,100 GDD to reach black layer, physiological maturity. For a hybrid that needs 2,700, its table gives V2 at 200, V6 at 475, V12 at 870, VT at 1,135, R1 at 1,400, R6 at 2,700. Another example in the same guide is a 116-day hybrid that reaches mid-pollination at 1,350 and black layer at 2,820. Your seed company's number for your hybrid is better than any table.

From what date do I start counting?

From planting for a crop, and for insects from the biofix, the date of a specific event such as the first trap catch. UC IPM stresses that a degree-day model must use the same method, thresholds and starting point as the research it came from; its codling moth model, for example, has a 50°F lower and 88°F upper threshold with a single sine method, which is different from the corn convention. Enter the temperatures from your start date forward, one day per line.

Where do I get daily high and low temperatures?

From your own thermometer, a nearby weather station, or the daily summaries published by the National Weather Service and your state climatologist or extension weather network. Use data from near your field, since temperatures differ from place to place. The calculator accepts up to 400 days, one line each, with the high first and the low second.

Can I use it for my garden?

Yes, with caution. Garden vegetables have their own base temperatures and maturity targets that vary by variety, and this page only has verified stage numbers for field corn. You can still enter a custom base, and a target total from a seed packet or extension publication, to see how far you are from it. For timing seed starts by frost dates, use the seed starting date calculator, and for how far apart to set plants, the plant spacing calculator.

Worked example: one week of corn

Seven days with highs and lows of 74, 52; 78, 55; 83, 58; 91, 63; 88, 61; 72, 49; 69, 47 (°F, example values, not a forecast). With a base of 50 and a limit of 86, the daily totals are 13.0, 16.5, 20.5, 24.5, 23.5, 11.0, 9.5, and they accumulate to 118.5 GDD, an average of 16.9 a day. The 91°F day counts as 86, and the cool weekend adds little. At that pace a hybrid in the Kentucky table would need roughly 12 days from planting to reach V2 at 200 GDD. In Celsius a day with a high of 30°C and a low of 10°C gives 10 degree days, and multiplying Celsius GDD by 1.8 converts them to Fahrenheit GDD.

Common mistakes

Mixing temperature scales: a base of 50 entered with Celsius temperatures counts nothing. Switching methods halfway through a season, so that one half of the total uses the simple average and the other the modified method. Starting from the wrong date (a calendar date instead of planting or biofix). Applying a corn threshold to a plant or insect with a different base; the 50°F base fits corn and many insects but not every crop. And counting the fractional day: a day with a low above the high, usually a typing error, would give nonsense, which is why the calculator rejects it.

Limits of this method

A daily high and low describe a day with two numbers; the actual temperature curve between them matters, and averaging can misstate a day when the weather swings widely. Sine-wave and triangle methods approximate the curve better, and UC IPM uses them for insect models, but they need thresholds and software that match the research. GDD also leaves out everything else that governs growth: moisture, soil temperature, day length, nutrients, disease and the hybrid itself. Stage totals are averages from field research and real fields vary by several days. Treat the stage table as a guide, and use your seed company's guidance for decisions about spraying, harvesting or planting.

Corn stages for a 2,700 GDD hybrid

Values from the University of Kentucky guide (Neild and Newman, 1987); hybrids differ, so ask your seed supplier for yours.

Sources, read October 27, 2026: University of Kentucky Cooperative Extension Service, AGR-202, “Corn Growth Stages and Growing Degree Days: A Quick Reference Guide” (Chad Lee), formula, examples and table 2 (after Neild and Newman, “Growing Season Characteristics and Requirements in the Corn Belt,” National Corn Handbook NCH-40, 1987); University of California Statewide IPM Program, “Degree-Days: About Degree-Days” and the codling moth degree-day table. For planning only; agronomic decisions should rest on your own field records and local extension advice. Related: seed starting date calculator, plant spacing calculator, Fahrenheit to Celsius, average calculator. How we check our calculators.